Fixed an error for the Coupled RHT-CFD Adjoint test in serial_regression_AD.py

[thomasdick]

The Coupled RHT-CFD Adjoint did not compare the computed gradient file to the reference file. Instead, the test case passed without file comparison.

Proposed Changes

Added a call to run_filediff to ensure the test is run.
Moved the Coupled RHT-CFD Adjoint test in serial_regression_AD.py down to the file difference test.

Related Work

This fixes the related issue #1431
The reference gradient file in the test case repo needs a small modification. See su2code/TestCases/pull/84

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[pcarruscag]

Thanks for fixing this 👍

[TobiKattmann]

💐 thanks Thomas.
As far as I see, there is a discrepancy for one DV only in that very Testcase, and that is in the last displayed digit (-0.00432385 vs -0.00432384) which I think is ok to just change. So please go ahead and change that. Once serial_regression_AD passes this can be merged from my perspective

[thomasdick]

Yes there is a discrepancy in the last digit for one the gradient values.
Since the reference file for the gradient values is in the Testcases repository and not the SU2 repository, I created a small pull request there to change the value. See su2code/TestCases/pull/84.
Together the changes lead to the test passing as expected.